Secretions of the GI Tract and Pancreas

Question 1

Acinar cells

Answer 1

Secrete initial saliva

Question 2

Myoepithelial cells

Answer 2

Have motile extensions

When stimulated by neural input, contract to eject saliva into mouth

Question 3

Intercalated duct

Answer 3

Saliva in the intercalated duct is similar in ionic composition to plasma

Question 4

Striated duct and ductal cells

Answer 4

Lined by columnar epithelial cells
Ductal cells modify the initial saliva to produce the final hypotonic saliva
They also modify concentrations of various electrolytes - actively absorb Na, passively absorb Cl, actively secrete K+
Ductal cells are impermeable to water

Question 5

Saliva composition and comparison to plasma

Answer 5

Saliva is composed of H2O, electrolytes, a-amylase, lingual lipase, kallikrein and mucus
It is hypotonic compared to plasma- has increased K+ and HCO3, decreased Na and Cl

Question 6

Parasympathetic innervation of salivary glands

Answer 6

Facial and glossopharyngeal nerves give off presynaptic nerves which synapse in autonomic ganglia and then give off branches to glands
Increases saliva production, HCO3 and enzyme secretions
Causes contraction of myoepithelial cells
Parasympathetic effect dominates

Question 7

Sympathetic innervation of salivary glands

Answer 7

Preganglionic nerves originate at the cervical ganglion, whose postganglionic fibers extend to the glands in the periarterial spaces
Same effects as parasympathetic, but is not used often

Question 8

Components of gastric juice

Answer 8

HCl- converts pepsinogen to pepsin
Pepsinogen
Mucus - helps neutralize acids with HCO3
Intrinsic factor
H2O

Question 9

Oxyntic gland area

Answer 9

Area of gastric mucosa
Located in proximal 80% of the stomach
Secretes acid

Question 10

Pyloric gland

Answer 10

Area of gastric mucosa in distal 20% of stomach (antrum)

Synthesizes and releases gastrin

Question 11

Cells in body of stomach

Answer 11

Parietal cells (secrete HCl and intrinsic factor)
Chief cells- secrete pepsinogen

Question 12

Cells in antrum

Answer 12

Mucus cells- secrete mucus, HCO3 and pepsinogen
G cells- secrete gastrin into the circulation (not into stomach. gastrin increases H/K ATPase activity and increases acidity)

Question 13

Parietal cells cellular mechanism of HCl secretion

Answer 13

HCO3/Cl antiporter pumps HCO3 into blood and chloride into cell
H/K ATPase antiporter pumps K into cell and H into stomach
Chloride follows H+ into stomach

Question 14

Non-parietal vs parietal gastric juice

Answer 14

Gastric juice is a mixture of non parietal and parietal juices
Non parietal is a basal alkaline secretion of constant and low volume consisting of K, Na and Cl
Parietal is slightly hyperosmotic and Cl is the only anion present (Cations include H+ of course)

Question 15

Vagus nerve effect on parietal cells

Answer 15

Causes ACh release which activates M3 receptor on parietal cells and the H/K ATPase increases secretion
Also causes GRP release which activates G cells to secrete gastrin- further activating parietal cells

Question 16

Effect of prostaglandins on H+ secretion

Answer 16

D cells secrete somatostatin which inhibits H+ secretion by acting on G cells
Prostaglandins also inhibit H+ secretion

Question 17

Omeprazole

Answer 17

Blocks H/K ATPase in stomach

Question 18

Atropine

Answer 18

Blocks ACh effects on M3 receptor, reducing H+ secretion (blocks vagal pathway)
Will not block vagal effects on gastrin secretion because the neurotransmitter at the synapse on G cells is GRP
It does block the ACh potentiated effects of histamine and gastrin though

Question 19

Cimetidine

Answer 19

Blocks H2 receptor from binding histamine and reduces H+ secretion
This blocks the direct action of histamine, but also blocks the potentiated effects of ACh and gastrin- because gastrin and ACh also activate histamine release

Question 20

Regulation of gastrin release by somatostatin and vagus

Answer 20

Somatostatin acts on G cells to inhibit gastrin release
Vagal stimulation causes gastrin release by releasing GRP and inhibiting release of somatostatin
Gastrin itself will increase somatostatin release as negative feedback system
H+ also triggers release of somatostatin

Question 21

Examples of potentiated effects: histamine and ACh

Answer 21

Histamine potentiates the actions of ACh and gastrin

ACh potentiates the actions of histamine and gastrin

Question 22

Cephalic phase of gastric secretion

Answer 22

Accounts for 30% of HCl secretion in response to meal
Simulated by smelling/tasting, chewing, swallowing
Vagus nerves stimulates parietal cells via ACh
Vagus nerve stimulates g-cells to release gastrin, further activating parietal cells
Produces enzymatic secretion
This phase is abolished by a vagotomy

Question 23

Gastric phase

Answer 23

Accounts for 60% of HCl released in response to a meal
Stimulated by stomach distension and presence of broken down proteins, AAs and peptides
Distension activates mechanoreceptors in mucosa of oxyntic and pyloric glands
Produces enzymatic secretion
Vagus nerve activates parietal cells via direct/indirect pathways

Question 24

Intestine phase

Answer 24

Accounts for 5-10% of HCl secreted in response to a meal
Distension of small intestine stimulates acid secretion
Digested protein stimulates acid secretion via direct effect on parietal cells and via gastrin secretion from intestinal G cells
Produces enzymatic and aqueous secretions

Question 25

What is most important stimulus for pepsinogen secretion

Answer 25

Vagus nerve stimulation | H+ triggers local cholinergic reflexes that stimulate chief cells to secrete pepsinogen

Question 26

Intrinsic factor

Answer 26

Required for B12 absorption in the ileum Secreted by parietal cells Failure to secrete IF is associated with achlorhydria and with absence of parietal cells

Question 27

Protective factors of gastric mucosa

Answer 27

HCO3 and mucus Prostaglandins Mucosal blood flow Growth factors

Question 28

Damaging factors of gastric mucosa

Answer 28

``` H+ and pepsin H. pylori NSAIDs Stress Smoking Alcohol ```

Question 29

Zollinger-Ellison syndrome

Answer 29

Large secretion of gastrin by duodenal or pancreatic neuroendocrine tumors Increases parietal cell mass, inhibits the absorption of sodium and water by the small intestine Creates ulcers due to excess H+ Low pH inactivates pancreatic digestive enzymes, interferes with emulsification, and leads to maldigestion and malabsorption (very high H+ secretion and gastrin levels)

Question 30

Pernicious anemia

Answer 30

Stomach does not produce enough IF, less B12 absorption Atrophic gastritis is chronic inflammation of stomach mucosa that leads to loss of parietal cells and can cause pernicious anemia Autoimmune metaplastic atrophic gastritis is when immune cells attack IF protein or gastric parietal cells

Question 31

Gastrectomy/Gastric bypass effect on B12

Answer 31

Gastrectomy causes loss of parietal cells | Gastric bypass excludes the stomach, duodenum and proximal jejunum which alters absorption of B12

Question 32

Secretin stimulation test

Answer 32

Used in diagnosis of gastrin secreting tumors Under normal conditions, secretin administration inhibits gastrin release In gastrinomas, injection of secretin causes a paradoxical increase in gastrin release

Question 33

Helicobacter pylori

Answer 33

Causes gastric and duodenal ulcers Releases cytotoxins that break down mucosal barrier and underlying cells Urease enzyme allows bacteria to colonize the mucosa by producing ammonia from urea and alkalizes the environment Ammonium results and causes cytotoxicity

Question 34

Gastric ulcer

Answer 34

Form on lining of stomach and caused mostly by defective gastric mucosal barrier, as opposed to increased acid secretion Increased gastrin levels because of decreased H+

Question 35

Duodenal ulcers

Answer 35

``` Form on lining of duodenum, more common Usually not malignant H+ secretion rates are higher than normal in response to food Increased gastrin levels Increased parietal cell mass ```

Question 36

Exocrine pancreas sympathetic/parasympathetic innervation

Answer 36

S- postganglionic nerves from celiac and superior mesenteric plexus innervate P- vagus nerve preganglionic fibers synapse in ENS, postganglionic fibers synapse in exocrine pancreas Parasympathetics activate, sympathetics inhibit

Question 37

Secretion of exocrine pancreas

Answer 37

Aqueous solution containing HCO3 Enzymatic secretion Contains acinar cells (enzymatic secretion) centroacinar cells and ductal cells (latter two types secrete aqueous solution that is alkaline rich)

Question 38

Modification of initial pancreatic secretion by ductal cells

Answer 38

Net result is secretion of HCO3 into pancreatic ductal juice and absorption of H+

Question 39

Cystic fibrosis and the pancreas

Answer 39

Pancreas is one of first organs to fail in CF CFTR is a regulated Cl channel in the apical surface of the duct cell Some CFTR mutations are associated with loss of HCO3 secretion, causing some loss of ability to flush out active enzymes from the duct May lead to acute and chronic pancreatitis

Question 40

CCK/Secretin regulation of pancreatic secretions

Answer 40

CCK induces release of pancreatic enzymes into the duodenal lumen Secretin induces the secretion of HCO3 from pancreatic cells into the duodenum